This invention relates generally to fuel cell assemblies, and more particularly to end plates for compressing a fuel cell stack therebetween.
Fuel cells electrochemically convert reactants, e.g., fuel and oxidant, to electricity. Unlike a battery, which contains a set amount of chemicals for generating electricity and which stops delivering electricity once the chemicals are consumed, a fuel cell can deliver electricity continuously so long as it receives fuel and oxidant. Fuel cells are generally categorized according to the type of electrolyte (e.g., solid oxide, molten carbonate, alkaline, phosphoric acid, or solid polymer) used to accommodate ion transfer during operation.
For example, a solid polymer electrochemical fuel cell generally comprises an MEA (membrane electrode assembly). The MEA includes a solid polymer membrane or PEM (proton exchange membrane) sandwiched between and in contact with two electrodes (one called an anode and the other called a cathode) made of porous, electrically conducting sheet material. The electrodes are typically made from carbon fiber paper or cloth. In addition, at the interface of the electrode and membrane, i.e., sandwiched therebetween, is a platinum-based catalyst layer to facilitate the electrochemical reaction.
Typically, the MEA is placed between two electrically conductive graphite plates which have one or more reactant flow passages impressed on the surface. The reactant flow passages direct the flow of a reactant to the electrode.
Fuel, such as humidified hydrogen, is supplied to the anode side of the fuel cell where the hydrogen reacts at the platinum-based anode catalyst layer to separate into hydrogen ions and electrons, as follows (anode reaction):
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The solid polymer membrane permits the passage of protons (i.e., H+ions) from the anode side of the fuel cell to the cathode side of the fuel cell while preventing passage therethrough of reactants (e.g., hydrogen and air/oxygen gases). The electrons migrate via an external circuit in the form of electricity.
Oxidant, such as humidified oxygen or air, is supplied to the cathode side of the fuel cell where it reacts at the platinum-based cathode catalyst layer with the hydrogen ions that have crossed the membrane and the electrons from the external circuit to form liquid water as a reaction product, as follows (cathode reaction):
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Additional cells can be connected together in series to form a fuel cell stack having increased voltage and power output. Such a fuel cell stack is typically provided with inlets, outlets, and manifolds for directing the flow of reactants (as well as coolant, such as water) to the individual reactant flow plates, and assembled between a pair of thick rigid end plates. The edges of the end plates are bolted together to apply a compressive force on the fuel cell stack.
One problem with end plates, and in particular service end plate for conducing one or more humidified reactants to the fuel cell stack, is condensation of water from the one or more humidified reactants as it passes through one or more openings extending through the service end plate. A reduction in the water content of the humidified reactant fluids can lead to drying out of the proton exchange membrane, and thus, reduction in the electrical output of the fuel cell stack.
Another problem with a fuel cell assembly having end plates is the loss of heat from the ends of the fuel cell stack. This results in the temperature of the fuel cell stack not being constant across the length of the fuel cell stack and the ends of the fuel cell stack not being maintained at the designed operating temperature of the fuel cell stack.
There is a need for inhibiting condensation of water from the one or more humidified reactants passable through one or more openings extending through a service end plate and/or a need for inhibiting heat loss from the ends of the fuel cell stack.
The above-identified needs are met by in one aspect of the present invention, an outermost heatable end plate for use in compressing a fuel cell stack in a fuel cell assembly in which the outermost heatable end plate includes a monolithic body having at least one flow channel. The at least one flow channel includes an inlet portion which opens onto at least one of an Inlet coolant manifold extending through the fuel cell stack and an outlet coolant manifold extending through the fuel cell stack.
In a second aspect of the present invention, a fuel cell assembly includes a fuel cell stack, a first outermost heatable end plate attachable to a second end plate for compressing the fuel cell stack therebetween, and wherein the first outermost heatable end plate includes a first monolithic body having at least one first flow channel. The at least one first flow channel includes an inlet portion which opens onto at least one of an inlet coolant manifold extending through the fuel cell stack and an outlot coolant manifold extending through the fuel cell stack.